Apparatuses for cooling heat generating spots of electronic devices have been used in a variety of forms, and cooling apparatuses which rely on air for cooling have been developed in a variety of configurations for purposes of simplifying the configuration. A plurality of forced air cooling type cooling apparatuses are provided in projector apparatuses which are projection display devices currently widely used for business and home applications.
A projection display device projects an image generated on an image display element of an image display onto a screen under magnification. Among such projection display devices, a liquid crystal projector apparatus which employs liquid crystal panels for image display elements displays an image on a screen in the following configuration and operation.
White light from a light source is reflected by a reflector, and is polarized or converted by PBS (Polarization-Beam Splitter) for separation into respective color light of red, green, and blue (R/G/B). Each separated color light is directed into each liquid crystal panel corresponding thereto, and optically modulated by the liquid crystal panel in accordance with a video signal. Each optically modulated color light is combined by a cross dichroic prism and projected onto a screen through a projection optical system.
In this event, a liquid crystal panel which operates in a TN (Twisted Nematic) mode can treat only a particular linearly polarized component, so that each color light is coordinated in a predetermined polarization direction (for example, S-polarization) by a polarizing plate before it impinges on the liquid crystal panel, and then optically modulated by the liquid crystal panel. Subsequently, an S-polarized component is cut by a polarizing plate on the exit side of the liquid crystal panel to extract a P-polarized component alone.
In this way, within a liquid crystal unit which includes an incident-side polarizing plate, a liquid crystal panel, and an exit-side polarizing plate, the incident-side polarizing plate and exit-side polarizing plate disposed before (upstream) and after (downstream) the liquid crystal panel along the optical axis each have functions of passing only polarized light in one axial direction and blocking other polarized light, and therefore generate heat during their operations due to light absorption. Also, the liquid crystal panel internally generates heat during its operations in the same manner because part of the transmitted light is blocked by a black matrix disposed on boundaries of respective pixels.
Organic materials are often used for these liquid crystal panels and polarizing plates, so that if they are irradiated with light at short wavelengths or are exposed to high temperatures for long time, their functions will be largely compromised by damaged alignment films of the liquid crystal panels, lower polarization selection characteristics of the polarizing plates, and the like. Accordingly, countermeasures to heat radiation, such as forced air cooling, are required for these components of the liquid crystal unit.
A specially configured cooling apparatus is required for efficiently cooling a plurality of heat generating spots of a plurality of components in an electronic device, each of which has surfaces that oppose each other with a spacing defined therebetween, and includes heat generating spots on the surfaces opposite to each other.
FIGS. 1(a) and 1(b) are schematic diagrams of the configuration of a liquid crystal projector apparatus of a background related art, where FIG. 1(a) generally illustrates the appearance of the general liquid crystal projector apparatus, and FIG. 1(b) illustrates the internal structure of the liquid crystal projector apparatus. FIG. 2 in turn is a schematic diagram of the internal configuration of the liquid crystal projector apparatus.
As mainly illustrated in FIG. 2, liquid crystal projector apparatus 1 includes cooling fan 3 for forcedly cooling liquid crystal unit 2, and cooling air duct 4, both of which are mounted in the housing of liquid crystal projector apparatus 1. In addition, lamp cooling fan 7 for cooling light source 5, power supply unit 10 and the like, exhaust fan 9 for exhausting the housing, and the like are provided as required.
Here, a general method of cooling liquid crystal unit 2 of liquid crystal projector apparatus 1 will be described with reference to FIGS. 3A and 3B. FIGS. 3A and 3B are schematic diagrams illustrating the configuration of a cooling unit for cooling the liquid crystal unit in the liquid crystal projector apparatus, where FIG. 3A is an exploded perspective view, and FIG. 3B is a schematic cross-sectional view for describing a forced air cooling operation.
In FIG. 3B, liquid crystal unit 2 including incident-side polarizing plate 12, liquid crystal panel 13, and exit-side polarizing plate 14 is provided for each color light (R/G/B), and air cooling device 15 including cooling fan 3 and cooling air duct 4 is disposed therebelow.
During the operation of air cooling device 15, air 16 from cooling fan 3 is fed into spaces among incident-side polarizing plate 12, liquid crystal panel 13, exit-side polarizing plate 14, which include each liquid crystal unit 2, from the lower end of liquid crystal unit 2 through discharge port 17 provided in cooling air duct 14 to perform forced air cooling.
In recent years, a reduction in size and an increase in luminance have been increasingly requested for liquid crystal projector apparatuses in accordance with their versatile usages. To respond to such requests, an increase in lamp power and a reduction in size of display device are now under progress, resulting in an increase in flux density of light incident on the liquid crystal unit, and a continuous increase in heat load on each component which forms part of liquid crystal unit 2.
For example, in a liquid crystal projector apparatus (1.0″-XGA) of 2000-lm class, a total amount of heat generated by liquid crystal units is approximately 15 W, while heat flux of an exit-side polarizing plate is approximately 0.6 W/cm2. However, with a 5000-lm class, a total amount of heat generated by liquid crystal units amounts to 35 W or more, while heat flux of an exit-side polarizing plate amounts to 1.4 W/cm2 or more.
Generally, when forced air cooling is used for cooling liquid crystal units, the amount of air fed by a fan is increased to enhance the air velocity around a heat generating spot to improve the heat transfer coefficient and heat radiation capabilities, thereby accommodating ever increasing heat loads.
However, as the amount of fed air is increased by increasing the rotational speed of the fan, operation noise will increase. On the other hand, as the amount of fed air is increased by employing a fan of larger size, this mitigates reducing the size of the apparatus.
On the other hand, requests have been gradually increased for longer lifetime of liquid crystal projector apparatus for reducing environmental loads and running cost. Apart from lamps, a factor which dominates the lifetime of the liquid crystal projector apparatus is mainly the lifetime due to degraded optical characteristics in the liquid crystal units. Accordingly, the lifetime can be increased by reducing the operating temperature of the liquid crystal units through improvements on the cooling performance.
However, in a laminar flow region, the average heat transfer coefficient of forced convection is proportional to the square root of the air velocity, while the operating temperature of the panel is reciprocally proportional to the square root of the air velocity. Thus, a reduction in the operating temperature of the panel to some extent will result in a lower sensitivity of a change in panel temperature to a change in air velocity.
FIG. 4 is a graph showing the relationship between a panel cooling air velocity and the panel operating temperature in 0.8″-SXGA (5000-lm class, 25° C. environment). For reducing the panel operating temperature from 70° C. to 60° C. (ΔT=−10° C.), the air velocity may be simply increased from 4.5 m/s to 8 m/s (ΔV=+3.5 as for reducing 60° C. to 50° C. (ΔT=−10° C.), the air velocity must be increased from 8 m/s to 18 m/s (ΔV=+10 m/s), as can be seen from the graph.
In this way, when forced air cooling is relied on to further reduce the panel operating temperature for prolonging the lifetime, the cooling air velocity is excessively required as the target temperature is lower. Accordingly, the fan operation noise can further increase, or the apparatus can be increased in size, as described above, and in some cases, the limit of cooling capabilities (air cooling limit) can be exceeded, so that the development of a highly efficient liquid crystal unit cooling system is an urgent necessity.
Further, in regard to the liquid crystal panel, another requirement exists for cooling from a viewpoint of image quality. Specifically, since the optical modulation effect to an input signal highly depends on the temperature in the optical modulation of the liquid crystal panel, thermal gradient on the panel plane cause variations in luminance and color, resulting in a degraded quality of projected images. For this reason, in cooling the liquid crystal panel, a cooling method is desired to minimize a temperature gradient and temperature variations which occur on the surface of the panel in operation.
FIG. 5 is a schematic side view illustrating a first related art example of liquid crystal unit cooling, as disclosed in Patent Document 1 (JP-11-295814A). Specifically, an apparatus for improving a liquid crystal panel cooling efficiency is provided with the aid of air directing plate 39 disposed below cross dichroic prism 35 to optimize the direction of air fed from cooling fan 3.
FIG. 6 is a schematic perspective view illustrating a second related art example of liquid crystal unit cooling, as disclosed in Patent Document2 (JP-2001-318361A). Specifically, an apparatus for improving a cooling efficiency is provided with protrusion 41 for guiding cooling air to liquid crystal holding a 40 to restrain interstices of air fed from duct discharge port 42.
FIG. 7 is a schematic cross-sectional view illustrating a third related art example of liquid crystal unit cooling, as disclosed in Patent Document 3 (JP-2004-61894A). Specifically, an apparatus is provided for adjusting a cooling air velocity by changing air passage widths (X and Y in the figure) between liquid crystal panel 13 and polarizing plate 14 with the provision of cutout 43 in an air passage of liquid crystal panel holding frame 40.
FIGS. 8A and 8B are schematic diagrams illustrating a first example of a fourth related art example of liquid crystal unit cooling, where FIG. 8A is a top plan view, and FIG. 8B is a lateral sectional view. FIGS. 9A and 9B are schematic diagrams illustrating a second example of the fourth related art example of liquid crystal unit cooling, where FIG. 9A is a top plan view, and FIG. 9B is a lateral sectional view. This fourth related art example is disclosed in Patent Document 4 (JP-2000-124649A).
In FIGS. 8A and 8B, air leading plate 44 having a U-shaped groove form is connected (positioned) between liquid crystal panel 13 and polarizing plate 12, for turning the direction of cooling air blown up from below on the exit side of liquid crystal panel 13 by the U-shaped groove at the upper end by 180° to feed the air from above to below on incident polarizing plate 12, thereby eliminating thermal gradient which occur on the panel surface.
On the other hand, in FIGS. 9A and 9B, an apparatus is disclosed for eliminating thermal gradient on a panel plane with the provision of a pair of cooling fans 3a, 3b above and below liquid crystal panel 13, with the liquid crystal panel interposed therebetween, to feed an air stream from below to above on the exit side and from above to below on the incident side.
FIG. 10 is a schematic lateral sectional view illustrating a fifth related art example of liquid crystal unit cooling. This fifth related art example is disclosed in Patent Document 5 (JP-2001-209126A) which discloses an apparatus which includes a liquid crystal panel cooling unit in a closed internal circulation structure with the use of circulation duct 45, where first cooling fan 3a air-cools a liquid crystal unit, exhaust air heated by received heat is transferred to external air circulation duct 46 shield by circulation duct 45 through a heat sink or the like (not shown, provided on the boundary of circulation duct 45 and air circulation duct 46 in the figure), and the heat of the heat sink is radiated by second cooling fan 3b which is provided outside.